The present invention relates to the field of integrated circuit memory technology. More specifically, the present invention provides techniques of evaluating characteristics of nonvolatile programmable memory cells and enhancing the reliability and service life of these memory cells.
Memory cells are used in the implementation of many types of electronic devices and integrated circuits. These devices include microprocessors, static random access memories (SRAMs), erasable-programmable read only memories (EPROMs), electrically erasable programmable read only memories (EEPROMs), Flash EEPROM memories, programmable logic devices (PLDs), field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), among others. Memory cells are used to store the data and other information for these and other integrated circuits.
As integrated circuit technology and semiconductor processing continue to advance, there is a need for greater densities and functionality in integrated circuits, which are often determined in a large part by the size of the memory cells. Further, it is desirable that the memory cells have improved operating characteristics, such as lower power consumption, nonvolatility, greater device longevity, improved data retention, better transient performance, superior voltage and current attributes, and improvements in other similar attributes.
There is a further need for improved techniques of evaluating the physical characteristics of nonvolatile memory cells. These physical characteristics or properties are important in the determination of an integrated circuit's service life and reliability. These measurements are also useful for study and use in improving memory cells. One property of a memory cell is margin, including program and erase margin, indicating the degree to which a cell is in a programmed or erased state. The degree of margin comes from a determination of the threshold voltages in the programmed and erased states. In particular, the voltage threshold (VTE) of erased nonvolatile memory cells such as EEPROM or Flash cells may be negative. Present techniques only allow threshold measurements in the positive range. Using these techniques, a device with a negative erased threshold voltage is presumed to have an erased threshold voltage of zero volts. However, the value of the erased threshold voltage is important in characterizing the reliability and service life of a memory cell, and it is important to be able to measure negative threshold voltage.
As can be seen, improved techniques for evaluating margin of memory devices are needed, especially when the memory devices have negative erased threshold voltages. Improved techniques are also needed for improving the reliability and longevity of these memory devices.